1. Field of the Invention
This invention relates to an incremental measuring system, particularly to a system for measuring lengths, comprising a scale member, which is provided with a scale, which is adapted to be scanned by an optoelectronic scanning unit for generating measured-value signals and is provided in at least one additional scannable track with two or more reference marks for a generation of reference signals at one or more selectable reference points, wherein the scale member and the scanning unit are encapsulated in an enclosure, which is formed with a slot, which is covered by sealing means and through which a member for actuating the scanning unit extends past said sealing means.
2. Description of the Prior Art
Measuring system of that kind are mainly used for a measurement of lengths with high accuracy and in a modified form as an angular position indicator for a measurement of angles with high accuracy. The scale consists in most cases of incremental elements consisting each of a bright field and a dark field with a pitch which may be as small as 0.004 mm. The scanning of the scale results in the generation of basically sinusoidal analog measured-value signals and a complete waveform of said signals is generated as a result of the scanning of a complete increment of the scale. In most cases at least two measured-value signals are produced, which may be displaced in phase, e.g., by 90.degree.. The measured-value signals are converted to countable signals and/or are fed to computers for interpolating computations so that measurements or indications having an accuracy of 0.0001 mm can be achieved. The direction of the scanning movement and the counting sense are determined in dependence on the phase-displaced signal trains. The countable digital signals may be used as control signals for use in control systems for machines or robots. To establish a definite relation between the analogue measured-value signals and that portion of the scale which is being scanned, it is necessary to set the evaluating means and the counting means to certain values, such as zero, when the scale is being scanned at preselectable points. Such setting operations must be performed before the measuring system is put into operation for the first time and in most cases at the beginning of each working day. For that purpose, reference signals are employed, which have been generated in response to the scanning of reference marks. The reference marks are provided on the scale member together with the measuring scale, in most cases by photolithography. Care must be taken that the proper phase relations are obtained between the reference signals and the analog measuredvalue signals which are generated and between the edges of the reference pulses and the digital countable signals which are derived from said measured-value signals. It may often be necessary to provide reference signals in response to the scanning at different locations on scale members which are equal in other respects. It would not be economical to manufacture each scale member with the specific reference marks which are required thereon. For this reason a practice which is often adopted resides in that a large number of reference marks having a uniform or non-uniform spacing are provided on the scale member as it is manufactured and one or more reference marks which is or are required are selected from the reference marks which have originally been provided. A removal of reference marks or a sticking of a covering on reference marks can usually be effected only in the plant in which the scale members are manufactured and in any case cannot be effected in most cases by the user of the encapsulated measuring system because such user would have to remove the scale member and to scratch off or cover certain reference marks and such work might often result in damage to the scale member or the scanning unit or in an ingress of foreign matter into the enclosure or in a change of the adjustment of the scanning unit. It must be borne in mind that the performance of measurements with high accuracy often requires the use of correction tables for the measuring system and such tables are prepared by the manufacturer of the measuring system as a result of a comparison of the results of the measurements performed by the measuring system with the results of measurements performed by a higher-ranking measuring system, such as a laser interferometer. Such tables may be stored in correcting memories of the evaluating unit. It may also be necessary to operate the measuring system in an inert gas atmosphere or under a superatmospheric pressure. In view of such difficulties, other methods have been adopted for a selection of reference marks used to generate reference signals. In one method which may be used, individual reference marks are identified by additional means, such as additional indentifying features, so that a reference mark which is being scanned can be identified and a reference signal will be transmitted only when it has been generated at a selected reference mark. Selecting systems which may be employed may consist of mechanical on-off switches for enabling an activating circuit only in response to scanning of preselected reference marks, or magnets which serve to operate reed relays may be provided on the scale member at preselectable locations. But the mounting of such magnets requires an ingress into the encapsulated measuring system. It is also known to provide the reference marks in a plurality of reference tracks, to provide a scanning unit in association with each reference track and to select a reference mark in that the scanning unit which is associated with the respective reference track is selectively enabled for a generation of reference signals. In that case too a change of the selection of specified reference marks and the adjustment of the selection requires considerable work and time and such work can be performed only by a skilled person and the means required for that purpose add to the costs of the system. When it is contemplated to erase reference marks which are not required, as stated hereinbefore, reference marks may be provided in two sets with a uniform spacing and duplicate scanning units may be arranged in an antiparallel connection so that their signals will offset each other unless an existing reference mark and a region in which a reference mark has been erased are scanned at the same time. In the previous practice an ingress into the measuring system has been required for an erasing of reference marks. Besides, reference marks which have been released cannot be reinstituted in practice. For this reason an initial selection of reference marks to be employed cannot subsequently be altered.